Summary I upgraded the X-axis of my Hesine M505 (Prusa i3) using http://www.thingiverse.com/thing:1103976, as the original did not have an X tensioner. I wanted to upgrade to a 16T GT2 pulley (https://www.amazon.com/gp/product/B0188HW4Z0/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1), but it would not fit in the idler mount. I redesigned the idler mount, based on the original but sized for the new pulley. This thing is the result.